The earthquakes that rattled Blackpool, England, in April and May wouldn't have attracted much notice in California. The strongest rated a mere 2.3 in magnitude.
But their possible connection to "fracking" raised eyebrows on both sides of the Atlantic.
The epicenter of one of the quakes lay less than 500 meters from a well used for hydro-fracturing, the process of pumping pressurized water and chemicals deep underground to crack rocks and release oil or natural gas. The company fracking the well halted operations and started studying the possible link.
As fracking has spread, most complaints have focused on the threat of groundwater contamination. But some opponents have also asked whether the process - which, after all, involves breaking subterranean rocks - could cause earthquakes as well.
The question is particularly pertinent in seismically active California, where the use of fracking appears to be growing. Fracking projects have been reported in Santa Barbara County and the Sacramento Valley.
Last week's 5.8 temblor in Virginia even prompted a round of speculation in the blogosphere that fracking could be to blame. There are, however, no fracking wells near the quake's epicenter or in any of the surrounding counties, according to the Virginia Department of Mines, Minerals and Energy.
Scientists say fracking does cause tiny earthquakes, but they're too small to be felt on the Earth's surface. The process can also cause quakes large enough to be noticed on the surface, but only if done near a fault line. And even then, the resulting tremors are likely to be mild.
"You're not going to get a really big earthquake," said Mark Zoback, a Stanford University geophysics professor who has studied the issue. "To get a big earthquake, you'd need a really big fault. When these oil fields are being developed, the companies are very aware of where the faults are. They don't want to do something stupid."
Some of the quakes that critics have blamed on fracking may have been caused by a related process - the disposal of the water used in fracking.
Similarity to air hockey
Many, but not all, fracking operations inject the wastewater deep underground. As the quantity of water builds up over time, it can change the pressure along a fault line, making the fault more likely to move.
"It's a little bit like an air hockey table," said Cliff Frohlich, associate director of the Institute for Geophysics at the University of Texas, Austin. "You pump air into an air hockey table so that when you push something, it will slip."
Frohlich was part of a team of researchers who studied a series of small quakes that struck near Dallas in 2008 and 2009, in an area where natural gas companies had used fracking. The epicenter turned out to be within a kilometer of a water disposal well, under the Dallas-Fort Worth International Airport. The largest quake measured 3.3.
"These earthquakes were not cases where frack jobs got out of hand," he said. "In a way, it was good news for the companies. If it's disposal that causes the quakes, you've got lots of options."
Those options include treating the water on the surface or shipping the water to a disposal well that isn't near a fault.
In July, Arkansas officials placed a moratorium on new disposal wells in a portion of the state shaken by hundreds of quakes, the largest of which reached magnitude 4.7. Four wells that had been used to dispose of water from fracking operations were shut down.
As fracking becomes more common in California, environmentalists want state officials to keep a close eye on any link to tremors.
A state bill that would force companies to disclose the chemicals they use in fracking originally contained a clause that would have required the companies to report whether the wells they planned to frack were near known fault lines. The clause, however, was taken out as a compromise with the oil and gas industry, said Renee Sharp, California director for the Environmental Working Group. She'd still like to see the state adopt that requirement.
"At least we'd know what was going on, so there'd be the possibility of really studying this issue," Sharp said.
Zoback says seismicity related to fracking and disposal wells is a manageable problem, one that shouldn't derail the growing production of gas from shale rock formations. Oil and gas companies need to identify faults near potential well sites, stay away from the faults, monitor for quakes during operations and stop work if quakes occur.
"I think we can replace coal with natural gas - I'm kind of militant about this," said Zoback, who recently served on a U.S. Department of Energy committee that recommended ways to protect the environment during fracking operations. "We've got to develop shale gas in an environmentally responsible way, and that means air and water quality, and it means not causing earthquakes."